Multi-stage flash evaporators

ABSTRACT

1,047,981. Multi-stage flash comparator; distilling sea water. RICHARDSONS, WESTGARTH &amp; CO. Ltd. Oct. 31, 1962 [Nov. 8, 1961], No. 40091/61. Heading B1B. In a flash evaporator, particularly for distilling sea water, comprising a series of stages, each comprising a flash chamber 14 and an associated heater-condenser 12 or 24, wherein the vapour is cpndensed, thus preheating the liquid to be evaporated, the vapour generated in one of the intermediate stages is withdrawn and compressed by a mechanical compressor 33, e.g. rotary compressor or piston compressor, and the compressed vapour, mixed with steam from an external source, is used to heat the brine in heater 20 to the required temperature before it enters the first flash chamber. Fresh cool brine, supplied to the evaporator via pump 28 and line 26,passes through the heatercondensers 24 and then via line 30 into the last flash chamber 14, where it mixes with the concentrated brine leaving the evaporator. Part of this brine mixture is discarded, andthe remainder is pumped to the first stage of the evaporator via line 10, heater-condensers 12,and heater 20. The vapour generated in flash chamber 14 passes into heater-condensers 12 or 24 via lines 13 or 25, and heats either the recirculated brine or the fresh brine. Vapour is withdrawn from heatercondenser 12a, or 24a or vapour receiver 40, via line 36, 38 or 42 respectively,by compressor 33, which is driven by steam turbine 32 receiving high pressure steam from boiler 31. The low pressure exhaust steam from turbine 32 is mixed with the compressed vapour from compressor 33, and the mixture is passed into heater 20 via line 22. The condensate from heater 20 may be removed for boiler feed water via line 21,or supplied to the condensate in receiver 12 of the first stage via line 23. The condensate formed in receivers 12 and 24 is finally removed by pump 16.

June 2l, 1966 R. STARMER MULTI-STAGE FLASH EVAPORTORSl Filed Nov. 5,1962 J7. orney m f M lv .S l w m I r S .e9 es do d e es e9 :S a Y h h hRQ h k, Ik. i /JLV 4 n WW QM .22E .SNE Jv mf m m m m m m m m ,M lv Tw Tlmw T MX 1 mm Q e F Q k s l .53 e@ JH e@ e3 eww MXHWMM w l 1 I S @N m n QS N NN 15 i i m mw m uw Nq N|\ M a Nm. Qw uw Si WT .f[f||| ....l l l lj' H United States Patent O 3,257,290 A MULTI-STAGE FLASH EVAPORATORSRoy Starmer, Northumberland, England, assignor to Richardsons,Westgartll & Co. Limited, Northumberland, England Filed Nov. 5, 1962,Ser. No. 235,359 Claims priority, application Great Britain, Nov. 8,1961,

9 Claims. (Cl. 202-173) This invention relates to multi-stage flashevaporators, particularly evaporators for obtaining distilled water fromimpure water or sea water.

In such evaporators the brine or liquid to be evaporated is passedthrough a series of heaters the last of which is generally heated bylive steam supplied from an external source. In many evaporators, landparticularly in ev-aporators for distilling sea water, the top brinetemperature to which the circulating brine may be heated in the steamheated chamber is limited by various operational and designconsiderations, and particularly by the onset of vscale deposition withhigher operating temperatures. In the case of evaporators for distillingsaline water such as sea water, this generally limits the permissibletop brine temperature to below 200 F., so that all the heaters in 4theevaporator operate at sub-atmospheric pressures.

This arrangement has the advantage that low grade waste steam can beused in the steam heated chamber. In some cases however, the only steamsupply available to the evaporator is steam under considerable pressure,and in these installations it therefore becomes desirable to providemeans which use this high pressure steam supply to the best advantage.

The present invention has been devised with this consideration in mind,and according to the invention a multi-stage flash evaporator comprisesthree or more stages each having aash chamber and an associated heateror vapour receiver, and a heater heated by live steam from an externalsource which is arranged to be traversed by the liquid to be evaporatedbefore it enters the ash chambers, in which the steam-hea-ted chamberhas an inlet connected to the delivery side -of -a mechanicalcompressor, such as a rotary compressor or piston compressor, the inletside of the compressor being connected to an intermediate stage of theevaporator so as to withdraw therefrom vapour of the liquid beingevaporated and deliver such vapour to the steam-heated chamber.

Preferably the said inlet or another inlet in the steamheated chamber isconnected to the delivery side of a steam tur-bine so that thesteam-heated chamber receives live steam which has'been expanded in theturbine. The turbine can then be used -to drive the compressor, and itis advantageous where this arrangement is used for the turbine and thecompressor to be so constructed and arranged that, during operation, thepressure of the exhaust steam from the turbine is approxi-mately equalto the pressure of the compressed vapour delivered by the compressor.

In order that the invention may be thoroughly understood, an evaporatorarrangement in accordance with it will now be described, by way ofexample, with reference to the accompanying drawing which shows thearrangement schematically.

' The flash evaporator shown in the drawing comprises a brine path 10which passes through a series of heatercondensers 12 ea-ch having a heatexchanger 12C therein wherein the brine is gradually brought to a highertemperature by the latent heat of condensation of vapour which is passedinto the heater-condensers 12 through passages 13 from a series of flashchambers 14. Dispassed from one heater condenser to another throughchamber to the next by conduits 10a.

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conduits 23a and then is removed by a distillate extraction pump 16, andthe liquid in the ash chambers 14 which is not evaporated is similarlyremoved from the final dash chamber, the liquid being passed from oneilash A circulating pump 18 is provided in the iiow path 10 forcirculating the brine. Y

A steam-heated chamber 20 `is provided which is traversed by the liquidpassing through heat exchanger 20a -tillate which condenses in theheater-condensers 12 is condensers 24 is possible.

immediately before it is passed into the flash chambers 14, and thissteam-heated chamber 20 is heated by steam supplied through the pipeline 22. Further heater-condensers 24 are supplied with vapour throughpassages 25 from the flash chambers forming the last few dashingoffstages, and these heater-condensers 24 are circulated by fresh coolliquid which serves to remove excess heat from the circulatory system.The fresh liquid ows along a path 26 which includes heat exchangers 24eand is circulated by a pump 28. After leaving the heater-condenser 24a,part of the slightly preheated stream of fresh liquid is discharged towaste while the remainder enters the final flash chamber 14 through apipeline 30. Here the fresh liquid mixes with unevaporated brine whichhas already been circulated along the iiow path 10, and the resultantmixture is withdrawn from the final flash chamber in order that part ofthe mixture may be discharged to waste while the other part isrecirculated to the heatercondensers 12 with the aid of the pump 18.Thus, all v the brine used for evaporation enters the evaporating systemthrough the pipeline 30.

As already indicated, an important aiml of the present invention is toutilise to the best advantage the pressure drop obtainable from theso-called live steam supplied from the external source 31. This isachieved by passing the steam supply through a steam turbine 32 beforeit reaches the steam-heated chamber 20, the -turbine being coupled to arotary compressor 33 so that the steam is used to compress a quantity ofbrine vapour which is withdrawn by the compressor from one of theintermediate stages of the evaporator. The compressed vapour is thenpassed, together with the exhaust steam from the turbine, into thesteam-heated chamber 20 where the steam and vapour mixture is condensed.The distillate thus formed in the steam-heated chamber 20 can be removedthrough the pipe-line 21 as boiler feed water or supplied through thepipeline 23 to the final heater-condenser 12. At the same time, thequantity of live steam required to operate the evaporator under acertain set of conditions can be reduced by a quantity correspondingapproximately to the quantity of vapour which is compressed in therotary compresssor 33. This produces an appreciable saving in steam andheat consumption which can vary between wide limits depending on thethermodynamic cycle used in the evaporator. Apart from the `turbine 32and the compressor 33, very little additional equipment or expenditureis required, and in fact a reduction in the heater surfaces in theheater Preferably, the turbine 32 and the compressor 33 are so designedthat the pressure of the exhaust steam from the turbine is approximatelyequal to the pressure of the compressed vapour delivered by thecompressor.

The quantity of vapour withdrawn from the intermediate stage by thecompressor 33 will depend, for any given live steam conditions, on thepressure in the evaporator stage from which the vapour is extracted andthe pressure in the steam-heated chamber 20 into which it is discharged.The ratio of vapour withdrawn to live steam supplied will riseconsiderably if the ratio of the higher to the lower of the twoabove-mentioned pressures is reduced, but on the other hand, thethermodynamic performance of the plant is best when the evaporator isoperated with as high a top brine temperature as possible and as low avapour temperature in the coldest heatercondenser 24h as is practicable.

It is to be noted that the brine vapour is withdrawn by the compressor33 from an intermediate stage of the evaporator and not from the firstor last stages. This is because we have found that a very considerableimprovement in the ratio of vapour withdrawn to live steam supplied andtherefore in the steam consumption of the plant, can be achieved if thecompressor 33 is made to withdraw vapour from a ash chamber or a heatercondenser operating at a pressure between the highest and the lowestpressures obtaining in the plant. The selected point of vapourextraction should preferably be the highest pressure freshheater-condenser 24a if more than one such heater is used, or the lowestpressure heater-condenser 12a. Broken lines 34, 36 and 38 are shown inthe drawing to illustrate the pipeline connections between the heatercondenser 24a or the heater condenser 12a and the suction side of thecompressor 33 if either of these arrangements is adopted. Alternatively,a specially arranged flash chamber 14a connected by a passage 39 to asimple vapour receiver 40 instead of to an associated heater can beused, the chamber 14a and its receiver 4d forming an intermediate stageof the evaporator, and the .receiver 40 being connected by the pipeline34 and a branch line 42 to the suction side of the ejector 32.

An investigation of the thermodynamics of the arrangements describedabove shows that no thermodynamic losses are involved in withdrawingvapour from the highest pressure heater-condenser 24a, since heat isstill rejected at that level from the plant, and the extraction ofvapour at an intermediate pressure increases the quantity of vapourhandled by the compressor 33. The same applies if the suction side ofthe compressor is connected to a special receiver 40 or its associatedflash chamber 14a arranged as described above. A certain thermodynamicloss is involved in connecting the suction side of the compressor 33 tothe coldest heater condenser 12a, but the loss is small as this heateroperates at a temperature level approaching the level at which heat isrejected altogether from the plant, and the thermodynamic loss may bemore than offset by gains resulting from improved performance of theturbine and compressor.

It may be expedient in some circumstances to withdraw vapour from aheater-condenser 12 operating at a pressure above that in theheater-condenser 12a. On the other hand, it may be found that theincreased quantity of vapour handled by the compressor 33 with such anarrangement does not offset the thermodynamic disadvantages followingthe extraction of vapour at a temperature level at which heat can beusefully transferred to the circulat- Y ing brine.

It may be found desirable to reduce the temperature of the brine leavingthe steam-heated chamber 20 below the maximum permissible limit given byconsideration of scale formation. In certain cases the reduction inthebasic net gained output ratio '(i.e. the ratio of the distillateproduced to the steam consumption) of the evaporator, due to suchreduction in temperature can be more than offset by the saving in steamconsumption due to the lower pressure rise required within thecompressor33. This applies particularly to sea water evaporators operating withfairly small temperature and pressure differences between the lowpressure and high pressure ends of the plant.

The advantages gained by means of the invention can be appreciated bycomparing the results obtained using :an evaporator of conventionaldesign with the results obtained after the same evaporator has beenmodified so as to be in accordance with the invention. Thus, in atypical evaporator having a gained output ratio of 8:1, each lb. ofsteam supplied direct to the heat input section would produce in theevaporator 8 lb. of vapour all of which would condense to form 8 lb. ofdistillate. The gross water putput would therefore be 9 lb. made up of llb. from the heat input section condensate plus 8 lb. of distillate. Butas l lb. of this would have to be returned to the boiler as feed water,the net output would be 8 lb. water from 1 lb. steam, which correspondsto the gained output ratio of 8:1.

If now the same evaporator ismodied so as to operate in accordance withthe invention, the following takes place. Each 1/3 lb. of steam passedthrough the turbine provides sufhcient energy to compress lb. of vapourin the compressor so that, when the turbine exhaust steam is mixed withthe compressed vapour, 1 lb. of heating steam is available to be fed tothe heat input section. This again produces 8 lb. of vapour in theevaporator, but as 2/3 lb. of this vapour is withdrawn to berecompressed, only 71/3 lb. condenses to form distillate. By adding tothis the 1lb. of steam condensed in the heat input section, the grossoutput of water is 81/3 lb. Of this quantity of 1/3 lb. is required forboiler feed water, so that the net output is, as in the operationdescribed above, 8 lb. However, as the steam consumed from the boiler isnow 1/3 lb. only, the effective gained output ratio is now 8:1/3

It is of course true that the steam supply to the turbine 32 now has tobe at a comparatively high pressure, but the additional fuel required togenerate this steam is insignificant compared with the saving resultingfrom the fact that the quantity of steam required has been reduced bytwo-thirds. Where the source of supply 31 takes the form of a boiler,the latter will be cheaper to construct and install due to its muchsmaller size.

Should the source 31 of high pressure steam become unavailable for anyreason, the plant shown in the drawing can easily be arranged to run ona low grade pressure steam supply by supplying the steam `direct to thesteamheated chamber 20 instead of through the turbine 32. In suchcircumstances the compressor can either be cut out of the evaporatingcycle or, alternatively, driven by some other form of prime mover.

It is to be understood that the connections between the turbo-compressorand the heater or heaters, and also between the turbine and the steamsupply source 31, include valves or other means for controlling thesteam supply and the withdrawal of the vapour, and for dealing with thevarious conditions encountered when starting up or shutting down theplant.

I claim:

1. A multi-stage ash evaporator comprising: a series of flash chambersincluding a first flash chamber, a plurality of intermediate fiashchambers and a last flash chamber, first feed conduit means connected tosaid first ash chamber, interconnecting conduits connecting said flashchambers in series to enable feed liquid introduced into said first ashchamber to flow therefrom through the series of Hash chambers to sai-dlast flash chamber, a first series of heater condenser chambers, asecond series of heater condenser chambers, flash vapor conduit meansconnecting each heater condenser chamber of said first series with vaporspace in a respective one of a first group of said flash chambersincluding said first flash chamber and several further flash chambersimmediately succeeding it in the flash chamber series, further ash vaporconduit means connecting each heater condenser chamber of said secondseries with vapor space in a respective one of a second group of saidash chambers including said last flash chamber and at least one furtherflash chamber immediately preceding it in the flash chamber series,drain conduit means to drain distillate from all said heater condenserchambers, a first plurality of heat exchangers disposed one in each ofsaid first series heater condenser chambers and interconnected in seriesto form a first heat exchanger fiow line for the ow of cooling fluidfrom one heat exchanger to another through the series, second feedconduit means connected to one end of said first heat exchanger flowline to deliver feed liquid thereto, first outlet conduit meansconnected to the other end of said first flow line, a steam-heatedchamber, a compressor, third feed conduit means delivering steam fromsaid compressor to said steam-heated chamber, a further heat exchangerdisposed in said steam-heated chamber and having an inlet connected tosaid first outlet conduit means, and an outlet connected to said firstfeed conduit means, whereby feed liquid flowing in said first flow lineis delivered to the s-eries of flash chambers by way of said furtherheat exchanger, a second plurality of heat exchangers disposed one ineach of said second series heater condenser chambers and interconnectedin series to form a second heat exchanger flow line for the flow ofcooling fluid, fourth feed conduit means connected to one end of, saidsecond heat exchanger ow line to deliver feed liquid thereto, secondoutlet conduit means connected to the other end of said second flowline, fifth feed conduit means connecting said second outlet conduitmeans to one of said second group ash chambers, and vapor conduit meansconnecting the inlet side of said compressor to vapor space in one ofsaid intermediate fiash chambers.

2. An evaporator according to claim 1, wherein the inlet side of thecompressor is connected to one of said first series heat condenserchambers that receives flash vapor from the `last flash chamber of saidfirst group of flash chambers.

3. An evaporator according to claim 1, wherein the inlet side of thecompressor is connected to one of said second series heater condenser`chambers that receives iiash vapor from the first ash chamber' of saidsecond group of fiash chambers. v

4. An evaporator according to claim 1, and comprising feed deliverymeans delivering feed liquid withdrawn from said last ash chamber tosaid second feed conduit means.

5. An evaporator according to claim 1, wherein condensed steam from saidsteam heated chamber is delivered to the first series heater condenserchamber that is connected to said first fiashchamber.

6. A multi-stage fiash evaporator comprising: a series of flash chambersincluding a first flash chamber, a plurality of intermediate ashchambers and a last flash chamber, first feed conduit means connected tosaid first iiash chamber, interconnecting conduits connecting said flashchambers in series to enable feed liquid introduced into said first ashchamber to flow therefrom through the series of flash chambers to saidlast ash chamber, a first series of heater condenser chambers, a secondseries of heater condenser chambers, flash vapor conduit meansconnecting each heater condenser chamber of said first series with vaporspace in a respective one of a first group of said flash chambersincluding said first flash chamber and several further fiash chambersimmediately succeeding it in the flash chamber series, further flashvapor conduit means connecting each heater condenser chamber of saidsecond series with vapor space in a respective one of a second `group ofsaid flash chambers including said last flash chamber, drain conduitmeans to drain distillate froml all said heater condenser chambers, afirst plurality of heat exchangers disposed one in each of said firstseries heater condenser chambers and interconnected in series to form arst heat exchanger fiow line for the flow of cooling fluid from one heatexchanger to another through the series, second feed conduit meansconnected to one end `of said first heat exchanger flow line to deliverfeed liquid thereto, first outlet conduit means connected to the otherend of said first fiow line, a steam-heated chamber, a compressor, thirdfeed conduit means delivering steam from said compressor to saidsteam-heated chamber, a

further heat exchanger disposed in said steam-heated chamber and havingan inlet connected to said first outlet conduit means, and an outletconnected to said first feed conduit means, whereby feed liquid flowingin said first fiow line is delivered to the series of fiash chambers byway of said further heat exchanger, a second plurality of heatexchangers disposed one in each of said second series heater condenserchambers and interconnected in series to form a second heat exchangerflow line for the flow of cooling fluid, fourth feed conduit meansconnected to one end of said second heat exchanger iiow line to deliverfeed liquid thereto, second outlet conduit means connected to the otherend of said second flow line, fifth feed conduit means connecting saidsecond outlet conduit means to one of said second groupfiash chambers,and vapor conduit means connecting the inlet side of said compressor tovapor space in one of said intermediate fiash chambers.

7. A multi-stage fiash evaporator according to claim 6 wherein saidsteam-heated chamber is connected to the delivery side of a steamturbine to receive live steam which has been expanded in said turbine,said compressor being driven by said turbine.

8. A multi-stage flash evaporator according to claim 6 wherein saidsteam-heated chamber is connected to the delivery side of a steamturbine to receive steam which has been expanded in said turbine, saidcompressor being driven by said turbine, and wherein said turbine andsaid compressor are so constructed and arranged that, during operation,the pressure of the exhaust steam from said turbine is approximatelyequal `to the pressure of the compressed vapour delivered by saidcompressor.

9. A multi-stage flash evaporator according to claim 6 wherein all theflash chambers supply ash vapor to a respective heater-condenser chamberwith the exception of one intermediate flash chamber, a vapour receiverbeing connected to vapor space in said one intermediate ash chamber andthe inlet side of said compressor being connected to the said receiver.

References Cited by the Examiner UNITED STATES PATENTS 2,759,882 8/ 1956Worthen et al. 2,893,926 7/ 1959 Worthen et al. 3,021,265 2/ 1962Sadtler et al 202-174 3,152,053 10/1964 Lynam 202 53 X NORMAN YUDKOFF,Pirmary Examiner.

M. H. SILVERSTEIN, F. E. DRUMMOND,

Assistant Examiners.

1. A MULTI-STAGE FLASH EVAPORATOR COMPRISING: A SERIES OF FLASH CHAMBERSINCLUDING A FIRST FLASH CHAMBER, A PLURALITY OF INTERMEDIATE FLASHCHAMBERS AND A LAST FLASH CHAMBER, FIRST FEED CONDUIT MEANS CONNECTED TOSAID FIRST FLASH CHAMBER, INTERCONNECTING CONDUITS CONNECTING SAID FLASHCHAMBERS IN SERIES TO ENABLE FEED LIQUID INTRODUCED INTO SAID FIRSTFLASH CHAMBER TO FLOW THEREFROM THROUGH THE SERIES OF FLASH CHAMBERS TOSAID LAST FLASH CHAMBER, A FIRST SERIES OF HEATER CONDENSER CHAMBERS, ASECOND SERIES OF HEATER CONDENSER CHAMBERS, FLASH VAPOR CONDUIT MEANSCONNECTING EACH HEATER CONDENSER CHAMBER OF SAID FIRST SERIES WITH VAPORSPACE IN A RESPECTIVE ONE OF A FIRST GROUP OF SAID FLASH CHAMBERSINCLUDING SAID FIRST FLASH CHAMBER AND SEVERAL FURTHER FLASH CHAMBERSIMMEDIATELY SUCCEEDING IT IN THE FLASH CHAMBER SERIES, FURTHER FLASHVAPOR CONDUIT MEANS CONNECTING EACH HEATER CONDENSER CHAMBER OF SAIDSECOND SERIES WITH VAPOR SPACE IN A RESPECTIVE ONE OF A SECOND GROUP OFSAID FLASH CHAMBERS INCLUDING SAID LAST FLASH CHAMBER AND AT LEAST ONEFURTHER FLASH CHAMBER IMMEDIATELY PRECEDING IT IN THE FLASH CHAMBERSERIES, DRAIN CONDUIT MEANS TO DRAIN DISTILLATE FROM ALL SAID HEATERCONDENSER CHAMBERS A FIRST PLURALITY OF HEAT EXCHANGERS DISPOSED ONE INEACH OF SAID FIRST SERIES HEATER CONDENSER CHAMBERS AND INTERCONNECTEDIN SERIES TO FORM A FIRST HEAT EXCHANGER FLOW LINE FOR THE FLOW OFCOOLING FLUID FROM ONE HEAT EXCHANGER TO ANOTHER THROUGH THE SERIES,SECOND FEED CONDUIT MEANS CONNECTED TO ONE END OF SAID FIRST HEATEXCHANGER FLOW LINE TO DELIVER FEED LIQUID THERETO, FIRST OUTLET CONDUITMEANS CONNECTED TO THE OTHER END OF SAID FIRST FLOW LINE, A STEAM-HEATEDCHAMBER, A COMPRESSOR, THIRD FEED CONDUIT MEANS DELIVERING STEAM FROMSAID COMPRESSOR TO SAID STEAM-HEATED CHAMBER, A FURTHER HEAT EXCHANGERDISPOSED IN SAID STEAM-HEATED CHAMBER AND HAVING AN INLET CONNECTED TOSAID FIRST OUTLET CONDUIT MEANS, AND AN OUTLET CONNECTED TO SAID FIRSTFEED CONDUIT MEANS, WHEREBY FEED LIQUID FLOWING IN SAID FIRST FLOW LINEIS DELIVERED TO THE SERIES OF FLASH CHAMBERS BY WAY OF SAID FURTHER HEATEXCHANGER, A SECOND PLURALITY OF HEAT EXCHANGERS DISPOSED ONE IN EACH OFSAID SECOND SERIES HEATER CONDENSER CHAMBERS AND INTERCONNECTED INSERIES TO FORM A SECOND HEAT EXCHANGER FLOW LINE FOR THE FLOW OF COOLINGFLUID, FOURTH FEED CONDUIT MEANS CONNECTED TO ONE END OF SAID SECONDHEAT EXCHANGER FLOW LINE TO DELIVER FEED LIQUID THERETO, SECOND OUTLETCONDUIT MEANS CONNECTED TO ONE END OF SAID SECOND HEAT EXCHANGER FLOWLINE TO DELIVER FEED LIQUID THERETO, SECOND OUTLET CONDUIT MEANSCONNECTED TO THE OTHER END OF SAID SECOND FLOW LINE, FIFTH FEED CONDUITMEANS CONNECTING SAID SECOND OUTLET CONDUIT MEANS TO ONE OF SAID SECONDGROUP FLASH CHAMBERS, AND VAPOR CONDUIT MEANS CONNECTING THE INLET SIDEOF SAID COMPRESSOR TO VAPOR SPACE IN ONE OF SAID INTERMEDIATE FLASHCHAMBERS.